Poly(heptazine imide) ligand exchange enables remarkable low catalyst loadings in heterogeneous metallaphotocatalysis

The development of heterogeneous metallaphotocatalysis is of great interest for sustainable organic synthesis. The rational design and controllable preparation of well-defined (site-isolated) metal/photo bifunctional solid catalysts to meet such goal remains a critical challenge. Herein, we demonstrate the incorporation of privileged homogeneous bipyridyl-based Ni-catalysts into highly ordered and crystalline potassium poly(heptazine imide) (K-PHI). A variety of PHI-supported cationic bipyridyl-based Ni-catalysts (LnNi-PHI) have been prepared and fully characterized by various techniques including NMR, ICP-OES, XPS, HAADF-STEM and XAS. The LnNi-PHI catalysts exhibit exceptional chemical stability and recyclability in diverse C−P, C−S, C−O and C−N cross-coupling reactions. The proximity and cooperativity effects in LnNi-PHI significantly enhances the photo/Ni dual catalytic activity, thus resulting in low catalyst loadings and high turnover numbers.

• Ni-PHI is mentioned in the main text but to understand its nature the reader should go to supplementary information. A very brief explanation in the main text would be appreciated. • In order to discard homogeneous catalytic processes, an additional experiment is necessary: After one catalytic run, the material is separated by filtration and the formation of products under the reaction conditions is discarded in the absence of material. • Some comments on the catalytic mechanism would enrich the manuscript. • The description of the experimental set-up in supplementary information should include not only the wavelength of light source (Blue LEDS) but also its intensity. Also, how constant temperature is granted in the photocatalytic experiment? The BET surface areas calculated for the K-PHI and bpyNi-PHI are 25.8 and 41.3 m 2 g −1 with total pore diameters of 10.3 and 9.94 nm, respectively. An increase in the BET surface area for bpyNi-PHI rationalizes the observed enhanced photocatalytic activity. The corresponding results ( Figure S3 and Table S5) were also added to the revised SI as follows: Figure S13. N2 adsorption/desorption isotherm (a) and BJH pore size distribution from the N2 adsorption branch (b) of K-PHI and bpyNi-PHI at 77 K. Table S5. BET specific surface area, pore diameter and total pore volume.

Sample
Specific surface area (m 2 g -1 ) Pore diameter (nm) Comment: Thank you for pointing this out. We performed the DFT calculation with 2 aqua ligands (bpy-Ni 2+ (H2O)2PHI 2-) and the optimized structure is shown in Figure S16 in Supplementary Information.

The conversion/yield obtained using semiheterogenous is PHI/NiLn was rather low;
however the reported analogue systems exhibit much better performance. Could the authors comment on this?
In our system, we tested the semiheterogenous PHI/NiLn system with lower LnNi loading (the same LnNi loading compared to our LnNi-PHI system, cat. 0.07-0.3 mol%), thus giving much lower yield. The reason as to why our LnNi-PHI system can give higher yield is the proximity and cooperativity of the LnNi2+ active species and the PHI photocatalyst carrier in LnNi-PHI might facilitate SET and free radical transfer (Table S7−S11).

Comment:
We have performed five different C-X bond formation catalytic reactions with aryl bromides instead of aryl iodides (1: 40%, 28: 52%, 3: 45%, 4: NR, and 5: trace). The results indicated that the aryl bromide is suitable for some C-X bond 5 formation reactions with lower reactivity. We believe the oxidative addition step may be the rate limiting step and the environment on Ni center in LnNi-PHI may lead to more difficult oxidative addition. We have included these results with related discussions in the main text ( Figure 6). "Additionally, strong electron deficient aryl bromide is also suitable for C-P, C-O and C-N couplings with lower reactivity (1, 3,  We thank the reviewer for the careful evaluation of our manuscript and the constructive suggestions clearly improved our manuscript.

Response to Reviewer #2:
This manuscript reports the use of Ni-functionalized Poly(Heptazine Imide) materials as photocatalysts in a variety of C−P, C−S, C−O, and C−N cross-coupling reactions.
The materials are carefully characterized, and the catalytic study comprehensively demonstrate the usefulness of the system reported. Therefore, I recommend its publication after consideration of the following minor aspects: Comment: Thank you very much for reviewing our manuscript. We appreciate the positive comments and constructive suggestions. The manuscript has been carefully revised based on your comments and suggestions.
Comment: Thank you for pointing this out. We have redrawn Figure 1 in the revised manuscript.

1s N XPS of the materials could be informative specially if compared with the pristine PHI material (Which is not presented).
Comment: The content of N in bipyridine is very low due to the limited bpyNi loading (6.0 wt %), which is difficult to obtain other useful information by analyzing the N 1s XPS spectrum of LnNi-PHI compared to that of K-PHI. The XPS studies of K-PHI results were also added in the revised SI ( Figure S6). and Ni(III) species, continuous blue LED irradiation is required to maintain high concentration of Ni(I) species. (Fig. 1e)."

The description of the experimental set-up in supplementary information should
include not only the wavelength of light source (Blue LEDS) but also its intensity. Also, how constant temperature is granted in the photocatalytic experiment?
Comment: Thank you for pointing these out. Irradiance of the LED modules was measured using CEL-NP2000 Optical Power and Energy Meter equipped. The output power at 3 cm distance from the light source 19 mW/cm 2 . In order to ensure that the reactions are run near indicated temperature, a simple cooling fan was used to aid in dissipating the heat generated from high power LEDs ( Figure S19). We have added these descriptions of the experimental set-up to the revised SI.